This invention relates to a multiple spindle kneading apparatus for improving ground by which a hardener is kneaded in soft ground to harden the soft ground, and to a ground improving method using this kneading apparatus.
Kneading machines have heretofore been used for hardening soft soil layers accumulated in bottoms of harbors, bays, lakes, rivers and the like by adding a hardener into these soft soil layers without shifting them from the accumulated positions and kneading the hardener into the soil layers under agitation. Since these soft soil layers are located deeply below the water surface, the total length of an agitation spindle provided with agitation vanes is 30 to 40 m. Accordingly, in the conventional kneading machines, agitation spindles shake violently during the operation and troubles are readily caused by collision of agitation vanes or excentric rotation of agitation vanes. In order to prevent such shaking of agitation spindles, the kneaders are often operated at a low speed. In this case, however, the agitation effect is drastically reduced.
Soft soil layers to be improved have much lower water contents than sludge deposit layers and hence, they have much higher viscosities. Further, the basic ground strength is relatively high in many cases. In conventional kneaders, the kneading zone is constructed by agitation vanes and projecting pilot blades. Accordingly, when the kneading zone is pierced into the soft soil layer, the soft soil, i.e., the soil to be improved, adheres to the pilot blades to wrap them. As hardening of the soil by the hardener is advanced, this soil adhesion phenomenon is enhanced and the amount of the adhering soil is increased. Therefore, the piercing speed is drastically lowered when the kneader is pierced in the soft soil layer and a large power is required for this piercing operation. In an extreme case, piercing per se becomes impossible.
In the conventional kneading machines, a hardener is injected to the kneading zone of the agitation spindle from (1) a hardener injection opening disposed above the kneading zone or (2) a hardener injection opening disposed below the kneading zone.
In case of (1), since the hardener is not supplied to the lower portion of the kneading zone, disturbed non-improved soil is left in the lower end portion of the improved soil layer and no sufficient supporting strength is given to the hardened improved soil layer. In case of (2), the soil layer is kneaded effectively with the hardener, but since by injection and kneading of the hardener the properties of the original ground are rapidly changed in a short time, the rotation load imposed on the agitation spindle is drastically increased and also a resistance to piercing or withdrawal of the kneading machine is enhanced.
As the conventional ground-improving method, there can be mentioned (a) a method in which a hardener is injected while a kneading machine is pierced into the ground and kneading is carried out when the kneading machine is pierced into the ground and when the kneading machine is withdrawn from the ground, and (b) a method in which a hardener is not injected while a kneading machine is pierced into the ground but is injected when the kneading machine is withdrawn from the ground and kneading is carried out during this withdrawal operation.
According to the method (a), the properties of the original ground are rapidly changed in a short time by injection and kneading of the hardener, and the rotation load imposed on the agitation spindle is drastically increased and also a resistance to piercing or withdrawal of the kneading machine is enhanced. According to the method (b), since kneading of soft soil with the hardener is performed when the kneading machine is withdrawn from the soil layer, a long time is required for completing the withdrawal operation and the operation efficiency is very low. Moreover, since soft soil is kneaded with the hardener when the kneading machine is withdrawn from the soil layer, no substantial kneading effect can be attained if the hardener is injected from the lower portion of the kneading zone. Therefore, the hardener must be supplied from the upper portion of the kneading zone, and disturbed non-improved soil is left in the lower end portion of the improved soil layer. As a result, no sufficient supporting strength is obtained and subsidence of the improved soil layer is caused to occur.